1. Field of the Invention
The present invention relates to a programmed automatic exposure control device, especially to an exposure device by which the effect of camera-shake is obviated so that a better quality image is obtained.
2. Description of the Related Art
To make it easier to take good photographs, many cameras today are provided with a programmed automatic exposure control device (hereinafter abbreviated as exposure control device) by which, according to a luminance of an object to be photographed, an aperture and a shutter speed are combined to automatically provide a correct exposure. A conventional exposure control device provided in an interchangeable lens camera is briefly explained as follows.
The prior art exposure control device has a plurality of program diagrams by which an aperture and a shutter speed are combined to obtain an appropriate exposure. FIG. 4 shows examples of three kinds of program diagrams, P.sub.1, P.sub.2, and P.sub.3. In FIG. 4, the vertical axis shows an A.sub.v value (aperture value), the lateral axis shows a T.sub.v value (shutter speed), and the oblique lines show equivalent exposure values.
In FIG. 4, if the program diagram P.sub.2 is taken as a standard, the program diagram P.sub.1 shows a line by which a photograph can be taken under a condition in which the shutter speed is raised at the same object luminance, and the program diagram P.sub.3 shows a line by which a photograph can be taken under a condition in which an aperture is stopped down at the same object luminance.
In the prior art, a program diagram used for exposure control is chosen from among the program diagrams shown in the drawing, according to the focal length of the interchangeable lens (a photographing lens) used for the photographing, and where a zoom lens is used in accordance with focal length changes caused by a rotation of the zoom barrel. Namely, in the prior art, since a program diagram is not provided for each photographing lens having a different focal length, in all cases in which a focal length of the photographing lens is longer than a predetermined value f.sub.1, the program diagram P.sub.1 is used, and in all cases in which a focal length of the photographing lens is shorter than f.sub.2 (wherein f.sub.2 &lt;f.sub.1), the program diagram P.sub.3 is used; in all the other cases, the program diagram P.sub.2 is used.
In the case of camera-shake, however, it is considered that this has very little effect at a shutter speed which is higher than a camera-shake limit speed value determined by a reciprocal number of a focal length of the photographing lens, wherein a dimension is ignored; i.e., the shutter speed is 1/50 sec when the focal length f=50 mm. With regard to this point, a conventional exposure control device does not have program diagrams for all photographing lenses having different focal lengths, but instead, a photographing condition is divided into zones according to a focal length, and program diagrams are allocated to each zone. Therefore, a point at which a shutter speed is lowest on the program diagram (a starting shutter speed; points shown by T.sub.v1, T.sub.v2, and T.sub.v3 in FIG. 4) is necessarily a common value, for example, of a plurality of photographing lens for which the program diagram is applied, and accordingly, the full ability of each photographing lens may not be properly utilized. Further, in the conventional program diagram, both the A.sub.v value and T.sub.v value change in accordance with a lowest starting shutter speed (called T.sub.vf) used as a starting point.